Oil exists in subterranean formations or reservoirs in a wide variety of forms, in a wide variety of formations and under a wide variety of natural conditions. In most cases natural forces present in the reservoir permit the production of significant amounts of the oil by so-called primary recovery methods. Usually this is brought about by the fact that reservoir pressure, supplied by gas under pressure, either in solution in the oil or as a gas cap, water, etc. is sufficient to force the oil to the surface of the earth. In any event, these so-called primary recovery methods are capable of recovering only minor portions of the original oil in place due to depletion of the natural forces and other factors. In some cases little or none of the oil can be produced by natural forces. Accordingly, a wide variety of supplemental or artifical recovery techniques have been employed and still more have been proposed in order to increase the recovery of oil from subterranean formations. If the artifical recovery technique is utilized in reservoirs having insufficient natural production forces it is often referred to as primary recovery and, if used immediately following discontinuance of primary recovery methods, such technique has been referred to as a secondary recovery technique. If a so-called secondary recovery technique is followed by another artifical recovery technique, the latter has often been referred to as tertiary recovery. However, the lines of demarcation among these three techniques have been obliterated to a certain extent and it is, therefore, best to refer to all such artifical recovery techniques, whether primary, secondary or tertiary, as "enhanced oil recovery" techniques. Irrespective of the name applied to the recovery technique, all such enhanced oil recovery techniques include the injection of a gaseous or a liquid fluid into one or more injection wells under a pressure sufficient to displace or drive at least a portion of the oil from the reservoir, i.e. above the reservoir pressure, and producing the thus displaced oil from one or more producing wells. Obviously, a wide variety of driving fluids or injection fluids and combinations thereof have been proposed. However, the basic drive fluids or injection fluids include air, natural gas, carbon dioxide, propane, steam, water, surfactants and polymers. Unfortunately, none of these materials is an ideal displacement fluid due to a number of factors which affect the amount of oil which can be recovered by enhanced oil recovery techniques.
It has long been recognized that the major factors which influence the amount of oil recovered by enhanced oil recovery techniques include the relative mobility of the reservoir oil and injected fluid, the wettability characteristics of the rock surfaces within the reservoir and the interfacial tension between the injected fluid and the reservoir oil.
Obviously, if plug-type flow of oil and displacing fluid from injection wells to production wells could be accomplished substantial amounts of the oil in place could be displaced. However, this is generally not accomplished because of the fact that most displacing fluids will travel faster through the reservoir than the oil because of adverse mobility ratios. While a rather simplistic explanation, the relatively low viscosity of gases, as opposed to the oil, causes the gas to follow paths of least resistance, with the result that the gas will channel through fractures and fissures, selectively pass through zones of higher permeability and in general contact a small area of the reservoir in passing from the injection well to the production well. In addition, gravity segregation of the injected gas and the oil causes the gas to rise to the top of the reservoir where it tends to ride over the top of the oil bank. Accordingly, while gases such as natural gas and air are usually readily available and relatively inexpensive, they are also relatively inefficient as displacing media under ordinary conditions. In addition, one must also consider the cost of compressing the gas to a pressure sufficient for displacement of the oil. On the other hand, liquids have a more favorable mobility ratio with respect to reservoir oil due primarily to their greater viscosity. Consequently, conventional water injection or waterflooding has been the most widely practiced enhanced oil recovery technique. However, the mobility ratio between water and reservoir oil is still generally poor. Accordingly, numerous modifications of conventional waterflooding have been proposed to overcome this problem. These include thickening the water with various materials, such as polymers, forming viscous water-oil emulsions by the use of surfactants, etc. Obviously, these thickening or emulsifying materials are expensive and cannot be used throughout the entire waterflood. Hence the thickening agent or emulsion is utilized only in that portion of the water in contact with the oil. An alternative is the injection of a small slug of polymer, generally having a viscosity greater than the viscosity of the oil, at the contact between the polymer and the oil, and a terminal viscosity, at the contact with the water, which is near that of the viscosity of water. Such graded concentration is usually logarithmic, from the viscosity of the reservoir oil to the viscosity of the water. In other variations, a thickening or viscosifying agent is preceded by one or more other displacing media and followed by water.
The wettability characteristics of the rock surfaces also affect displacement of oil by water. If the rock surfaces are oil wet, substantial amounts of the oil will adhere to the rock surfaces and resist displacement by the water. If the oil wettability of the rock surfaces can be altered either by decreasing the oil wettability or even reversing the wettability, to render the rock surface water wet, substantial improvement in oil displacement by water can be attained. Such reduction of oil wettability or reversal of wettability can also be accomplished by the utilization of surfactants. However, as previously discussed, such surfactants are expensive and therefore must be utilized in limited quantities, generally as a slug ahead of the water drive.
The interfacial tension between a displacing fluid and reservoir oil is primarily dependent upon the ability of the two materials to mix. As a result miscible replacement techniques have been developed. For example, if natural gas is compressed to a sufficiently high pressure, usually above about 3000 psi, the gas can be rendered miscible with the reservoir oil. However, in some cases, if the miscibility pressure is too high to be practical or the reservoir cannot withstand pressures of this magnitude, this process cannot be used. In addition, the additional compression cost also adds to the cost of the project. Where applicable, however, this technique has proven quite effective. Similarily, at lower pressures, carbon dioxide can be rendered miscible with reservoir oils and miscible displacement can be carried out by displacing the oil with carbon dioxide. In addition to the advantage of the lower miscibility pressure, carbon dioxide has the advantage of a relatively high solubility in water. Consequently, techniques have been proposed in which a slug of carbon dioxide is followed by water or the carbon dioxide is dissolved in the water. At still lower pressures ethane and propane and mixtures thereof can be made miscible with reservoir oil. However, these materials, particularly propane, are expensive relative to the value of the oil displaced and accordingly can not be utilized in unlimited amounts. As a result the "propane slug process" has been developed in which a slug of propane is driven through the reservoir by gas, usually natural gas, under conditions such that the propane is miscible with the oil being displaced and with the driving gas. Again, while this technique is effective in appropriate reservoirs, utilization of gases as the drive fluid interjects the above-mentioned problems of mobility. Finally, under certain conditions, surfactants can be utilized in the miscible displacement of oil. At this point it should be recognized that the terms "miscible" and "miscibility", as they relate to enhanced oil recovery techniques, have been somewhat misused, for example by the use of terms such as "partial miscibility". However, what is generally meant by such terms is that one fluid is partially soluble in the other. Consequently, a more accurate definition of "miscible" or "miscibility", and the definition which will be utilized herein, is that the two fluids in question are mixable with each other in all proportions and of "solubility", that there is a limit to the amount of a material which is soluble in or will mix with a fluid. While miscibility between the reservoir oil and the displacing fluid can be said to be ideal, to the extent that the oil-water interfacial tension is minimal, it is not necessary to obtain miscibility in order to reduce the oil-water interfacial tension and substantially improve displacment of oil by the drive fluid. Significant lowering of oil-water interfacial tension can be accomplished by the utilization of surfactants and highly effective immiscible displacement can be attained.
It is obvious from the above that the utilization of surfactants in enhanced oil recovery techniques has numerous advantages over the other techniques discussed. As previously indicated, the surfactant reduces the interfacial tension between a surfactant solution and reservoir oil and alters the oil wettability of the rock surfaces, thus substantially improving displacement of the oil. Secondly, since the surfactant solution is a liquid, it can be driven by water and the disadvantages of unfavorable mobility ratios, which are present when gases are used as drive fluid, are significantly reduced. Finally, enhanced oil recovery techniques utilizing surfactants can be utilized in reservoirs which have already been subjected to other recovery techniques, particularly where the reservoir has been produced to its economic limits by waterflooding. As a result, a substantial amount of research has been carried out in developing a wide variety of techniques utilizing surfactants and in improving the basic forms of these techniques. As previously indicated, because of the relative cost of surfactants, the surfactants are generally utilized in small amounts or in slug type operations in which the surfactant solution is driven through the reservoir by water.
The most basic of the surfactant techniques involves the injection of an aqueous surfactant solution, simply to reduce the oil-water interfacial tension. Such techniques are often referred to as "low tension waterflooding" techniques. Today one of the most promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonates, having a predetermined equivalent weight range, under controlled conditions of salinity. This basic technique is further improved by sequential injection of a protective slug, the surfactant slug, a mobility control slug and finally water. The protective slug is an aqueous solution of sodium chloride which is injected in order to displace reservoir water ahead of the subsequently injected surfactant slug. The protective slug is substantially free of divalent ions which would tend to precipitate the subsequently injected surfactant. The surfactant slug comprises an aqueous solution of petroleum sulfonates and contains sodium chloride in a concentration, typically between about 1.0 to 7.0 weight percent, which will promote the desired low interfacial tension between the injected water and the reservoir oil. The subsequently injected mobility control slug is a thickened water slug containing a viscosifier or thickening agent, such as a water soluble biopolymer or polyacrylamide. The mobility control slug is preferably of logarithmically graded concentration in order to provide an initial viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. Finally, the driving fluid may be water from any source, but is usually brine present in the reservoir with the oil. In addition to petroleum sulfonates, a wide variety of synthetic sulfonates and complex sulfonates derived from either petroleum or synthetic sources have been proposed to further improve the process and overcome other problems which exist in certain reservoir environments.
As previously indicated, surfactants may be utilized under conditions to produce miscible or immiscible displacement of the oil. In addition, such surfactants have been used in systems which do not form microemulsions and those which do form microemulsions. In recent years considerable research has been devoted to the latter systems.
The microemulsions which have been proposed have been selected from compositions in the single phase region of a ternary diagram. Such microemulsion systems can be either oil-external microemulsions or water-external microemulsions. When such microemulsion systems are used, it is believed that the initial stages of oil recovery involve an efficient miscible displacement with subsequent immiscible displacement, upon the breaking down of the microemulsion into multiple phases due to dilution of the microemulsion with crude oil and reservoir water at its leading edge and dilution with the aqueous drive fluid at its trailing edge. Hence, optimization of such microemulsion surfactant systems is approached in terms of minimization of the multiphase region in the phase diagram so as to prolong miscible displacement with low interfacial tensions in the multiphase regions to thereby enhance immiscible displacement. From a practical standpoint, however, the development of effective microemulsion systems which can economically recover oil from a subterranean formation suffers from certain drawbacks in that it is difficult to maintain miscible displacement and it is difficult to obtain the low interfacial tensions necessary to provide effective immiscible displacement after miscible displacement ceases.
Surfactant systems have been developed which form microemulsions on contact with the reservoir oil. For example, U.S. Pat. No. 3,373,809 discloses recovering oil through the formation of a microemulsion formed in situ by injecting a surfactant system. This patent is based on the formation of a single phase microemulsion system with the reservoir oil by injecting a surfactant system to form the microemulsion system in situ. However, in order to achieve the desired results, extremely high concentrations of surfactant must be utilized. Such quantities of surfactant are usually in excess of about 7% to 15% by weight so as to provide a composition within the single phase region of a ternary diagram and, as such, can easily exceed the value of the oil recovered. Accordingly, it is becoming well recognized that it is impractical from an economic standpoint to maintain such a highly concentrated surfactant composition in the reservoir, which will remain effectively miscible throughout the lifetime of the operation, as proposed by the above patent and others.
Recent work has led to the suggestion of injecting microemulsion systems wherein the microemulsion phase is immiscible with the resident fluids in the reservoir. For example, U.S. Pat. No. 3,885,628 proposes to form a multiphase microemulsion system above ground by mixing oil, brine and surfactant and injecting at least the immiscible microemulsion phase. In some cases this patent suggests injecting one or more of the other phases, which exist in equilibrium with the microemulsion phase along with the immiscible microemulsion phase. Later work, as set forth in U.S. Pat. No. 3,981,361, describes procedures for producing surfactant systems above ground which are injected as an immiscible microemulsion. In this case emphasis is placed on the injection of the single immiscible surfactant-rich microemulsion phase. Also, U.S. Pat. No. 3,938,591 discusses the injection of immiscible microemulsion systems which resist uptake of oil and water into the immiscible microemulsion phase. In the last three techniques described, there is the obvious disadvantage of requiring the injection of a composition containing substantial amounts of oil which, of course, adds to the cost of the injected composition. In addition, there is the problem of achieving the optimum system for a given oil, since it turns out that different oils behave differently.
In order to overcome the above-mentioned and other difficulties encountered in the prior art use of surfactants in oil recovery, U.S. Pat. Nos. 4,079,785 and 4,125,156, which are incorporated herein by reference, disclose that an effective immiscible surfactant drive can be carried out by injecting a slug of surfactant solution comprising a surfactant, an electrolyte, preferably a monovalent metal electrolyte, usually sodium chloride, water and, optionally, a cosurfactant to form a multiphase system in situ in the reservoir which comprises; at least two different regions, for example, an oil-rich region and a microemulsion region. The latter patent points out that best results are obtained when three different multiphase regions are formed, namely, a microemulsion, in equilibrium with an oil phase, a microemulsion in equilibrium with both an oil phase and a water phase and a microemulsion in equilibrium with a water phase. It is pointed out in this patent that among the variables which affect the three-phase region in which a particular system will partition are salinity, oil type, surfactant average equivalent weight, cosurfactant type and temperature. The patent also goes on to point out that, if all variables are fixed except the salinity, the system will shift from a microemulsion in equilibrium with on oil phase to a microemulsion in equilibrium with both an oil phase and a water phase to a microemulsion system in equilibrium with a water phase, as the salinity increases from zero. Finally, the patent sets forth a simple procedure which can be carried out in a laboratory to establish the system of water, electrolyte, surfactant and, optionally, cosurfactant and the proportions thereof which will be most effective for enhancing oil recovery when injected into the reservoir of interest. This laboratory procedure involves equilibration of water from the reservoir of interest or synthesized reservoir water, the surfactant, and optionally the cosurfactant, and oil from the reservoir of interest, a synthesized oil from the reservoir of interest or pure hydrocarbons or mixtures thereof having an equivalent alkane carbon number matching that of the reservoir oil at differing electrolyte concentrations. The optimum system which would be most effective for enhancing oil recovery is that system which will form the second phase, i.e., a microemulsion in equilibrium with both an oil phase and a water phase over a narrow range of salinity. This optimum salinity can, of course, occur at different electrolyte concentrations, depending upon the characteristics of the water and the oil.
Any of the surfactant systems previously discussed, whether those which form a microemulsion or those which do not or those which are miscible or those which are immiscible with the resident fluids, can contain varying amounts of monovalent metal electrolytes, either as components of the formation water, as added electrolytes or both. In some cases the salinity of the surfactant system, which is injected into the reservoir may range up to 4.0%. In addition, the so-called "high brine" reservoirs contain formation waters with substantial amounts of monovalent metal salts and/or polyvalent metal salts or ions, such as calcium and magnesium ions, in concentrations as high as 20,000 parts per million in the case of the polyvalent metal ions. Such high brine environments limit the usefulness of most surfactants, since such surfactants lack stability in such environments. Specifically, the surfactants tend to precipitate in the presence of monovalent salts such as sodium chloride at concentrations in excess of about 2 to 3 weight percent and in the presence of polyvalent metal ions, such as calcium and magnesium ions, at concentrations of about 50 to 100 parts per million and above. Such precipitation of the surfactants not only reduces the amount of surfactant available for lowering interfacial tension and altering rock wettability but the precipitates, in some cases, will eventually plug the formation. While a wide variety of anionic and nonionic surfactants have been proposed as surfactants in oil recovery techniques and the latter are generally more tolerant to high brine environments than the former, surfactants used predominately to date have been petroleum sulfonates and synthetic alkyl or alkylaryl sulfonates. While these surfactants are comparatively inexpensive, are readily available and are extremely effective in reducing interfacial tension to desired low values within the millidyne per centimeter range, they are also the least stable in high brine environments. In any event, most such surfactants can be satisfactorily utilized only if the calcium and magnesium concentration of the formation water is below about 500 parts per million.
In view of the limitations imposed on the use of certain surfactant types in high brine environments, various amphoteric surfactants which are stable in high brine environments have been proposed. For example, a mixture of sulfonated betaine, an alkyl or alkylaryl sulfonate and a phosphate ester sulfonate, amphoteric quaternary ammonium carboxylates, certain hydrocarbyl quaternary ammonium sulfonates or carboxylates, a mixture of an anionic surfactant, such as an alkyl or alkylaryl sulfonate, a water soluble salt of an alkylpolyethoxylated sulfate, an anionic surfactant, such as sodium dodecylpolyethoxy sulfate, and a nonionic surfactant such as a fatty acid diethanolamide, mixtures of a quaternary ammonium sulfonate with a C.sub.5 -C.sub.8 aliphatic alcohol, etc. Obviously, most of these proposed surfactants or mixtures increase the cost of the operation and, in the case of mixtures, the proportions of the components are highly critical. It has also been suggested that the detrimental effect of polyvalent metal ions can be avoided by the addition of a catalytic agent to the floodwater or by preflooding the reservoir to displace the divalent salt-containing brines and thereby eliminate the problem. Here again, the addition of other materials to the recovery system will increase the cost of the operation and manipulative techniques, such as preflooding, unduly extend the time necessary to obtain recovery and the cost.